For me the one I can't go without is a Swiss army knife. Pick your flavor.
For me, it is the Explorer Plus:
Two blades, two openers with screwdriver tips, scissors, magnifier, inline Phillips, corkscrew with eyeglass screwdriver, awl, hook, pen, pin, toothpick and tweezers.
I'll deal with the soft, non-locking blades in exchange for the tools.
There are many that could be on this list, but to stick to the thread I will say an alox Victorinox in a flavor of your choosing.
Agreed. Same reason I am mulling over another Rat 1.Opinel to remind you how a knife is supposed to cut.